Cap space alone won't net you a top 6 forward outside of free agency. You need to be able to trade from strength to fill needs. I'll readily admit that I am not familiar with the Habs prospects - but having traded Segachev, what other high quality prospects does Montreal have to offer now?
Arizona and Toronto are arguably the most loaded teams in terms of forward prospects. Short of trading multiple high picks, I don't think you have anything they'd take for any of Kapanen, Leivo, Brown, Keller, Strome or Domi.
You have a serviceable top 4 Dcore. If Mete pans out, you guys are in good shape for about 45 mins of a game, but that projected bottom pairing is dicey at best. Granted, I'm only well read about Montréal, I would rather watch about 20 other teams play. But I think they reached trading Sergachev, they are going to miss him.
I'm on the fence about Drouin being a significant upgrade on Radulov. I think that your team would look better had it used your vaunted cap space to re-sign Radulov and kept Sergachev to improve your blue line. Drouin may be a 1C, at worst he's a top line winger. Radulov was definitely a top line winger. Dynamic, good for possession and good on the power play.
Proof? You think for a second Yzerman looked at Montréal's cap situation before agreeing to the Drouin trade? Montréal got Drouin because Tampa was in a cap bind, had to make a move, and Montréal had a blue chip prospect available. And they needed to sweeten the deal with a conditional pick, which if I had any say in it Tampa ensures Sergachev plays under the 40 games to get Montréal's 2nd (via WSH) in exchange for Tampa's 6th.